When it rains on a parking lot, a road, or other impervious surface, rain water will not permeate into the ground as it would if the surface were in its natural condition. The rain water will instead run off this surface often discharging directly into a stream or receiving body. Typically, some form of rain water collection and/or diversion is incorporated into the design of a paved surface of sufficient size to warrant storm water control. This results in an accumulation of the rain water prior to discharge into the watershed.
Because impervious surfaces typically have been paved for a purpose, they will have vehicles, activity and/or traffic on them, which will cause an accumulation of pollutants between rain events. The rain water runoff therefore will include a the accumulated pollutants as they are washed from the impervious surface during a rain storm. Treatment of rain water runoff is important to the preservation of watersheds. The pollutants are typically at their highest concentration in the rain water during the first portion of a storm, as most of the pollutants are typically washed off in the initial, and usually less intense, part of a storm. Consequently, the first runoff water is the most critical to treat. In an effort to minimize the impacts of pollutant contaminated runoff water, various systems have been developed to treat runoff water including removing the pollutants by separation and/or filtration.
Because precipitation occurs at variable rates, a system must be able to treat runoff during low rain flow as well as during high rain flow periods. The system must have the capacity to capture and treat polluted runoff while having the ability to properly handle large water flows which exceed the in line treatment capacity of a system, without release of pollutants or untreated contaminated water into the watershed during high rain flow events. Consequently, treating storm water creates additional difficulties because the system must be able to clean the water yet be able to pass very intense storms without flow slowdowns or backups that could cause flooding.